With a mild cam, 3.23 gears and a four speed we managed around 15mpg in my old Dart hardtop. That was also with an old AVS not fully tuned. I think the same combo is capable of near 20mpg on the highway with the proper parts...
i had a 4-speed manual 4door dart big heavy son of a gun all iron 340 9:1 compression rally wheels with a .551 lift solid cam and a thermoquad that consistently got 19mpg highway
with lighter wheels and engine parts i probably could have gotten 20mpg